The present invention generally relates to a process for the pyrolysis of waste materials particularly medical waste. More particularly, the invention relates to a pyrolysis process, wherein the waste material is placed in a sealed container. The sealed container is inserted in a load chamber and the waste material is subjected to the process of pyrolysis.
In recent years, government agencies, industries, and other organizations have had to address various problems relating to the handling and processing of organic waste materials including chemical and biological products. The disposal of medical waste is a particularly difficult problem, because of the presence of infectious bacteria, viruses, and other pathogens in the waste. It has been found that heating such organic waste materials to extremely high temperatures causes the components to thermally decompose. The heat energy converts the chemical components of the waste material (primarily carbon, hydrogen, and trace elements) to gases. A pyrolysis process is commonly used to thermally decompose and chemically transform the waste materials.
The term, xe2x80x9cpyrolysisxe2x80x9d, can have different meanings depending on its context. For example, xe2x80x9cpyrolysisxe2x80x9d is defined as the xe2x80x9ctransformation of a compound into one or more substances by heat alone, i.e., without oxidation.xe2x80x9d (Hawley""s Condensed Chemical Dictionary, 13th Ed. (1997).) In the Code of Federal Regulations (CFR) setting forth standards for the performance of hospital/medical/infectious waste incinerators, xe2x80x9cpyrolysisxe2x80x9d means xe2x80x9cthe endothermic gasification of waste materials using external energy.xe2x80x9d (40 C.F.R. xc2xa760.51c) Typically, in commercial pyrolysis operations, waste material is loaded into a pyrolysis furnace or chamber, and there is generally some small amount of air (oxygen) present in the furnace. There can be several reasons for the presence of the air in the furnace. Some air may enter the furnace during the loading of the waste in the furnace chamber as the door to the chamber is opened and closed. Also, some air may be entrained within the waste. Further, the pyrolysis furnace may be operated at a slight negative pressure resulting in a small amount of air being drawn into the furnace through deficient seals. Thus, the term, xe2x80x9cpyrolysisxe2x80x9d, is commonly used in the industry and used herein to encompass processes, wherein the atmosphere in the pyrolysis furnace may at times contain a very small amount of air (oxygen) during the pyrolysis reaction, but the amount is so small as to preclude the presence of visible combustion.
For industrial applications, the pyrolysis of the waste materials is typically a first step in the overall destruction of the materials. The pyrolysis process volatilizes or gasifies the organic compounds found in the waste and produces exhaust gases containing volatile organic compounds. In a second step, a burner unit combusts or oxidizes the volatile organic compounds.
Pyrolysis furnaces should not be confused with incinerators that operate in a starved-air mode. Such incinerators typically include primary and secondary combustion chambers. In the incineration process, a burner or other ignition source produces an open flame in the primary chamber. Combustion air is supplied to the primary chamber at a rate which is less than the stoichiometric amount of oxygen required to achieve complete combustion of the volatile organic compounds evolved from the thermal decomposition of the organic waste materials. Then, in the secondary combustion chamber, excess combustion air is supplied to completely decompose and oxidize the waste exhaust gases. Lewis, U.S. Pat. Nos. 4,474,121 and 4,517,906 disclose methods and apparatus for controlling the addition of an auxiliary fuel to a two-stage combustion furnace system which is operated in a starved-air mode in the first stage and in an excess air mode in the second stage. One problem with such starved-air incinerators is that the open flame in the primary combustion chamber produces turbulence and causes the suspension of particles in the exhaust gas stream. The particulate passes through the secondary combustion chamber and is emitted as pollutants, unless additional pollution control systems (e.g., scrubbers) are employed. It is expensive to install such air pollution control systems on incinerators, but such systems are often necessary to meet emission standards.
As discussed above, pyrolysis processes for destroying waste materials are generally known in the industry. For example, Hansen et al., U.S. Pat. No. 5,868,085 discloses a waste treatment unit having: a main frame; an input stage through which the waste material to be treated is introduced through an arrangement of valves that can be controlled to prevent unwanted incorporation of air or oxygen into the pyrolytic process; and a pyrolytic assembly comprising a thermally-insulated outer housing coaxially surrounding an ellipsoidally-shaped pyrolytic chamber. A rotatable screw conveys waste through the retort as the pyrolysis reaction takes place. A heating chamber is defined as the space between the outer housing and the retort. Fuel gases are combusted within the heating chamber to provide a source of heat energy for the pyrolysis. According to the ""085 Patent, the gases liberated from the feed material during pyrolysis are processed to draw off pollutants contained therein by a combination of condensation and thermal oxidation. The gases are then either vented to the atmosphere or routed to supply energy, such as to a steam generator.
Keough, U.S. Pat. No. 4,648,328 discloses an apparatus and process for the pyrolysis of used vehicular tires. The apparatus includes a reaction chamber. According to the ""328 Patent, tire fragments are introduced into and removed from the reaction chamber through airlock mechanisms to prevent the ingress of ambient air as the fragments are conveyed through the chamber by a chain and flight conveyor. The process includes shredding the used tires, preheating the tire fragments, passing the fragments through the reaction chamber, separating solid and gaseous products, and recycling a portion of the gaseous product to the heating means.
Also, incinerator processes, which introduce a flame into the incinerator chamber to burn the waste, are known. Brookes, U.S. Pat. No. 4,603,644 discloses an incinerator having a receiving chamber with an opening (vent) in a rear wall. An ignition chamber is supplied with fuel and air and fires a flame down onto the biomass placed in the chamber. The opening in the receiving chamber leads to an afterburner chamber having a burner member which bums the volatile constituents in the gases from the receiving chamber. The afterburner chamber transfers the heat to ducts which occupy the space under the receiving chamber, a heat transfer chamber.
One problem with the foregoing processes is that firing the burner in the chamber can cause instability and turbulence leading to the emission of particulate and ash material. These materials may be emitted from the system as pollutants. Accordingly, there is a need for a pyrolysis process, wherein a flame is not introduced in the pyrolysis chamber to thermally decompose the waste. One object of the present invention is to provide such a pyrolysis process.
In addition, Brookes, U.S. Pat. No. 5,611,289 discloses a gasifier for gasifying biomass waste. The gasifier comprises a primary chamber for receiving the waste, a fume transfer vent, and a mixing chamber to accept the pyrolysis gases from the primary chamber. The fumes then flow to an afterburner chamber, where a burning flame oxidizes the constituents of the fumes. According to the ""289 Patent, a partitioning wall is disposed between the flame chamber and the primary chamber so as to preclude the heating flame from entering the chamber. A heat transfer chamber accepts the fully oxidized fumes, and heat from the fumes causes the heat transfer chamber to be heated. The primary chamber has a heat conductive floor and is superimposed on the heat transfer chamber. The heat from the heat transfer chamber rises through the floor to heat the primary chamber and biomass waste.
However, one disadvantage with the foregoing, conventional pyrolysis process is that transferring heat through the floor of the primary chamber is a relatively slow process. Thus, there is generally a long time period required for raising the temperature in the primary chamber and completing the pyrolysis reaction. This time-consuming process can be costly and inefficient.
Another disadvantage with the above-described conventional pyrolysis process is that depending upon the type of waste, it may not be possible to reach the required temperature in the primary chamber even if heat is applied through the floor for a long period of time. To overcome this limitation, the door to the primary chamber has a small air inlet allowing a small amount of air to enter the chamber. The introduction of air raises the temperature of the chamber by means of combustion of the waste material. Once combustion occurs, the process becomes exothermic and is no longer a pyrolysis process.
Further, the afterburner chamber is always in fluid communication with the heat transfer chamber and the hot gases always pass through the heat transfer chamber without control. Thus, heat is continuously transferred into the primary chamber so long as the auxiliary heat input burner in the afterburner chamber is burning. This results in two potential problems: 1) volatile organic compounds can be produced in the primary chamber before the afterburner chamber has reached the proper operating temperature resulting in incomplete combustion and emissions; and 2) highly volatile waste may evolve volatile organic compounds at such a high rate that the primary chamber will exceed acceptable temperatures, thus driving the volatilization rate ever faster to excess temperature limits and excess emissions.
In view of the foregoing problems with conventional pyrolysis processes, there is a need for a system, wherein the transfer of hot gases from the oxidation chamber to the pyrolysis chamber can be conducted in a controlled manner. If desired, the hot gases should be capable of being transferred to the pyrolysis chamber rapidly to heat the waste materials. One object of the present invention is to provide such a pyrolysis process. These and other objects, features, and advantages of the present invention are evident from the following description and figures.
The present invention relates to a process for the pyrolysis of waste material, particularly medical waste. In general, the process comprises the following steps. The waste material is placed in a sealed pyrolysis container, and the container is inserted into a load chamber. The discharge port of the container is connected to a pyrolysis gas transfer duct so that the container is in fluid communication with an oxidation chamber. The discharge port should be connected to the pyrolysis gas transfer duct by a mechanical locking means to form a substantially air-tight seal.
The load chamber holding the pyrolysis container is heated so that heat is transferred into the container causing the waste materials to decompose and produce pyrolysis gases comprising volatile organic compounds. The pyrolysis gases flow from the pyrolysis container, through the pyrolysis gas transfer duct, and into the oxidation chamber. The pyrolysis gas transfer duct may contain an air inlet port for maintaining the pyrolysis container at a negative pressure and adding air flow for initial pyrolysis gas combustion at the inlet to the oxidation chamber.
The oxidation chamber includes a burner unit and at least one air inlet port for controlling air flow into the oxidation chamber. The burner unit is located in the upper portion of the oxidation chamber and produces a flame for preheating the oxidation chamber and maintaining the required temperature for combustion of the pyrolysis gases. The oxidation chamber typically comprises multiple air inlet ports. Particularly, the oxidation chamber may contain tangential air inlet ports for directing air tangentially into the chamber, and radial air inlet ports for directing air radially into the chamber. In the oxidation chamber, the pyrolysis gases are combusted and heat is produced. At least a portion of the heat produced in the oxidation chamber is directed through a hot gas transfer duct, and into the load chamber.
The hot gas transfer duct contains at least one hot gas control damper. A microprocessor may be used to control the hot gas control damper and regulate the amount of heat directed to the load chamber. The microprocessor may use an algorithm including a time/temperature profile, combustion air input rate, and burner input rate to determine the endpoint of the process.
Different pyrolysis containers may be used. In one embodiment, the container has an integrated structure comprising four sidewall panels, a base panel, a cover, and a discharge port. The container may be made from a high temperature-resistant metal alloy and include a high temperature-resistant gasket for sealing the cover. The container may be introduced into the load chamber by means of transport guide rails. In another embodiment, the pyrolysis container includes a rectangular-shaped recessed portion, wherein the recessed portion extends upwardly from the base panel to provide a core heating surface. Various other sealed pyrolysis containers having different geometries and designs may be used in accordance with this invention.